**Core Concept**
Cullen's sign is a clinical sign characterized by a bluish discoloration of the periumbilical skin, resulting from subcutaneous hemorrhage. This phenomenon is associated with the retroperitoneal spread of blood from a severe intra-abdominal hemorrhage, often seen in the context of acute hemorrhagic pancreatitis.

**Why the Correct Answer is Right**
Acute hemorrhagic pancreatitis is a severe form of pancreatitis characterized by extensive necrosis and hemorrhage within the pancreas. As a result of the high pressure and inflammation, blood from the pancreas can spread to the adjacent tissues, including the peritoneal cavity and the subcutaneous tissue around the navel. The subcutaneous hemorrhage causes the characteristic bluish discoloration of the skin, known as Cullen's sign. The periumbilical region is particularly prone to this sign due to the presence of the inferior epigastric vessels.

**Why Each Wrong Option is Incorrect**
**Option A:** Acute cholecystitis is an inflammation of the gallbladder, which can cause right upper quadrant abdominal pain but is not typically associated with Cullen's sign.
**Option B:** Acute pancreatitis can cause abdominal pain and can sometimes lead to hemorrhage, but it is not typically characterized by the specific finding of Cullen's sign.
**Option D:** Blunt injury abdomen can cause a range of injuries, including hemorrhage, but Cullen's sign is not a specific indicator of this condition.

**Clinical Pearl / High-Yield Fact**
Cullen's sign is a classic example of a "sentinel sign" that can indicate the severity of an intra-abdominal hemorrhage. It is essential to recognize this sign in a timely manner to initiate prompt treatment and prevent further complications.

**✓ Correct Answer: C. Acute hemorrhagic pancreatitis**